Marine seismic explorations usually employ seismic sensors below the water's surface, e.g., in the form of streamers towed behind a ship or cables resting on the ocean floor. A typical seismic streamer includes multiple sensors positioned at spaced intervals along its length, and often many such streamers or cables positioned in parallel lines over the survey region.
An underwater seismic wave source, such as an air gun, produces pressure waves that travel through the water and into the underlying earth. When such waves encounter changes in acoustic impedance (e.g., at a boundary or layer between strata), a portion of the wave is reflected. The waves reflected from subsurface layers are called “seismic reflections”. The seismic streamers or cables provide an array of seismic sensors to detect these seismic reflections and convert them into signals for storage and processing.
One notable consequence of operating in the marine environment is the presence of “ghost reflections” caused by pressure wave reflections off the water's surface. The downward-moving ghost reflections can interfere with the sensors' measurements of the upward-moving seismic reflections, causing substantial amplitude enhancements at some frequencies (due to constructive interference), and reductions at other frequencies (due to destructive interference).
To address this issue, the industry developed the usage of dual sensors at each sensing node. A pressure sensor (“hydrophone”) and a velocity sensor (“geophone”) provide measurements of pressure and (directional) velocity that, when appropriately combined, enable ghost reflections to be filtered out of the survey data. Such techniques are standard in the industry. Accordingly, seismic explorationists have come to expect both types of sensors to be available when specifying parameters for acquiring seismic survey data.
There exists certain technologies that offer potential advantages for conducting long-term seismic monitoring and/or seismic data acquisition in extreme marine environments. As one example, efforts have been made to develop optical seismic sensors that demonstrate high reliability, have long lifetimes, and do not require any electrical power. Such results have resulted in the creation of optical hydrophones and accelerometers, but to date the author is aware of no satisfactory optical geophones.